Not new that I'm aware of unless you're going through prepaid. You're looking at $200-$250 minimum for full retail typically. If you need to stay under $100 you're probably better off going refurbished or through a third party.

Not much. It really depends though. You definitely want to double check if the refurbished are covered under warranty, since some aren't, and I would look up reviews for refurbished from your carrier or the company that refurbishes their phones, but aside from the warranty and specific companies that do poorly, they're usually held to decent standards.

not going to happen. Just to clue you in, the iPhone5 costs $600-$800 in a retail setting.

There is no way you will ever find a decent smartphone for less than $100 without signing a contract to get it subsidized. Heck, all the parts in, say, and Galaxy Nexus cost close to $200, without counting labor, design, shipping, advertising, R&D, etc.

Even couple year old smartphones that are alirght are still going to be well over that in a retail setting.

Your only chance at under $100, off contract, is to find someone selling a used one stupid cheap, or that is beat up big time.